1. The Field of the Invention
The invention involves methods and systems for hydroprocessing heavy oil feedstocks that include a significant quantity of asphaltenes and fractions boiling above 524° C. (975° F.) to yield lower boiling, higher quality materials. The invention specifically relates to fixed bed hydroprocessing methods and systems that employ a colloidal or molecular catalyst and a porous supported catalyst, and methods for upgrading an existing fixed bed system, to better handle lower quality feedstocks and inhibit formation of coke precursors and sediment and/or extend the life of the supported catalyst.
2. The Relevant Technology
World demand for refined fossil fuels is ever-increasing and will inevitably outstrip the supply of high quality crude oil, whether as a result of actual shortages or due to the actions of oil cartels. In either case, as the price or shortage of crude oil increases there will be an every-increasing demand to find ways to better exploit lower quality feedstocks and extract fuel values therefrom. As more economical ways to process lower quality feedstocks become available, such feedstocks may possibly catch, or even surpass, higher quality crude oils, in the not-too-distant future, as the primary source of refined fossil fuels used to operate automobiles, trucks, farm equipment, aircraft, and other vehicles that rely on internal combustion.
Lower quality feedstocks are characterized as including relatively high quantities of hydrocarbons that have a boiling point of 524° C. (975° F.) or higher. They also contain relatively high concentrations of sulfur, nitrogen and metals. High boiling fractions typically have a high molecular weight and/or low hydrogen/carbon ratio, an example of which is a class of complex compounds collectively referred to as “asphaltenes”. Asphaltenes are difficult to process and commonly cause fouling of conventional catalysts and hydroprocessing equipment.
Examples of lower quality feedstocks that contain relatively high concentrations of asphaltenes, sulfur, nitrogen and metals include heavy crude and oil sands bitumen, as well as bottom of the barrel and residuum left over from conventional refinery process (collectively “heavy oil”). The terms “bottom of the barrel” and “residuum” (or “resid”) typically refer to atmospheric tower bottoms, which have a boiling point of at least 343° C. (650° F.), or vacuum tower bottoms, which have a boiling point of at least 524° C. (975° F.). The terms “resid pitch” and “vacuum residue” are commonly used to refer to fractions that have a boiling point of 524° C. (975° F.) or greater.
By way of comparison, Alberta light crude contains about 9% by volume vacuum residue, while Lloydminster heavy oil contains about 41% by volume vacuum residue, Cold Lake bitumen contains about 50% by volume vacuum residue, and Athabasca bitumen contains about 51% by volume vacuum residue. Resid contains even higher concentrations of fractions that boil at or above about 343° C. (650° F.), with vacuum tower bottoms almost exclusively comprising fractions that boil at or above about 524° C. (975° F.).
Converting heavy oil into useful end products requires extensive processing, including reducing the boiling point of the heavy oil, increasing the hydrogen-to-carbon ratio, and removing impurities such as metals, sulfur, nitrogen and high carbon forming compounds. Examples of catalytic hydrocracking processes using conventional supported catalysts to upgrade atmospheric tower bottoms include fixed-bed hydroprocessing, ebullated- or expanded-bed hydroprocessing, and moving-bed hydroprocessing. Noncatalytic processes used to upgrade vacuum tower bottoms include thermal cracking, such as delayed coking and Flexicoking, and solvent extraction. Solvent extraction is quite expensive and incapable of reducing the boiling point of the heavy oil. Existing commercial catalytic hydrocracking processes involve rapid catalyst deactivation and high catalyst cost, making them currently unsuitable for hydroprocessing vacuum tower bottoms unless substantially diluted with lower boiling fractions, such as atmospheric tower bottoms. Most existing ebullated bed processes operate at less than 65 wt % conversion, while most fixed bed processes have less than about 25 wt % conversion.
A major cause of catalyst and equipment fouling is the undesired formation of coke and sediment, which often results when asphaltenes are heated to the high temperatures required to effect catalytic and thermal cracking. Supported catalysts used in commercial hydrocracking processes such as fixed-bed and ebullated-bed processes utilize solid supported catalysts that include clusters of catalytic sites located within pores or channels in the support material. Most heavy oil feedstocks contain a significant portion of asphaltene molecules, which are either too large to enter the pores of the catalyst support or else become trapped within the pores. Asphaltene molecules that become trapped in the pores deactivate the catalyst sites in the blocked pores. In this way, smaller asphaltene molecules can progressively block all catalyst sites, entirely deactivating the catalyst.
Moreover, larger asphaltene molecules form free radicals, just like other hydrocarbon molecules in the feedstock, but, unlike smaller molecules in the feedstock, are too large to enter the catalyst pores. Because of this, they are generally unable to react with hydrogen radicals located at the catalyst sites. As a result, the larger asphaltene free radicals are free to react with asphaltene and other free radicals in the feedstock, thereby forming larger molecules which continue increasing in size that can foul both the catalyst and the hydroprocessing equipment through the formation of coke precursors and sediment. The tendency of asphaltenes to form coke and sediment increases as the conversion level of the residuum increases due to the more strenuous conditions required to increase conversion. The undesirable reactions and fouling involving asphaltene greatly increase the catalyst and maintenance costs of ebullated-bed and fixed-bed hydrocracking processes. They also render existing commercial processes unsuitable for hydroprocessing vacuum tower bottoms and other very low quality feedstocks rich in asphaltenes.
Exacerbating the relatively low conversion levels using fixed bed hydroprocessing systems is the inability to proportionally convert the asphaltene fraction at the same conversion level as the heavy oil as a whole. Even though ebullated bed hydroprocessing systems are able to operate at substantially higher conversion levels than fixed bed systems, disproportional conversion of asphaltenes relative to the heavy oil as a whole is also problem with ebullated systems. The result of disproportional conversion is a progressive buildup of asphaltenes in the processed feedstock, with an attendant increase in the likelihood that coke and sediment will form in the reactor and other processing equipment.
Another problem involves the formation and continued reaction of free radicals outside the porous supported catalyst within a fixed bed reactor. In general, the beneficial upgrading reactions occur within the pores of the supported catalyst where the active metal catalyst particles are located. Asphaltenes and other larger molecules contained within heavy oil that are too larger to enter the pores of the solid catalyst may form free radicals that are unable to be capped with hydrogen and which can therefore react with other free radicals to yield even larger molecules. They can also form coke and sediment within the reactor, which can plug up or otherwise foul the reactor and/or porous supported catalyst, leading to a pressure drop within the fixed bed reactor.
Another issue unique to fixed bed hydroprocessing systems is the need to remove heat that is generated as the feedstock travels down through the catalyst bed and undergoes hydroprocessing reactions. Unlike ebullated bed reactors in which there is continuous mixing of the feedstock, catalyst and hydrogen, the static nature of fixed bed reactors leads to localized temperature buildups or overheating that might cause undesirable reactions (e.g., equipment and/or catalyst fouling). Cool hydrogen is typically injected into reactor between the layers of catalyst in order to cool the feedstock to prevent overheating and undesirable reactions this might cause.
Another problem associated with conventional fixed bed hydrocracking processes are generally low conversion levels, which are due in part to the tendency of the catalytic activity of the supported catalyst to diminish over time. Moreover, the tendency of asphaltenes or other large molecules within a heavy oil feedstock to foul the fixed bed reactor and/or the need to control the temperature within the fixed bed reactor as discussed above require operating the reactor at relatively low conversion levels (e.g., typically below about 25%) to prevent fouling of the equipment and/or porous supported catalyst. Once the catalytic activity of the catalyst is diminished, the only way to increase catalytic activity is to replenish the old catalyst with new catalyst, which generally requires the complete shut down of the fixed bed reactor at considerable cost. Even with newly replenished catalyst, the conversion level of typical fixed bed systems is substantially lower than ebullated bed systems.
In view of the foregoing, there is an ongoing need to provide improved fixed bed hydroprocessing systems and/or improve (i.e., modify) existing fixed bed systems to overcome one or more of the foregoing deficiencies.